1. Field of the Invention
The present invention relates to a power transmission device and a waveform monitor circuit for use in the power transmission device.
2. Description of the Related Art
In recent years, a contactless power transmission method (also referred to as wireless power transmission method) which enables power transmission without a contact of a metal portion by utilizing electromagnetic induction has attracted lots of attention. Japanese Laid-Open Patent Application Publication No. 2009-33955 discloses a prior art of such a contactless power transmission method. This prior art discloses a waveform monitor circuit in which, in a case where data communication is performed from a power reception device (secondary side) toward a power transmission device (primary side), a signal (coil end signal) induced by a primary coil based on load modulation at the power reception side, is taken out and rectified, thereby generating a waveform monitor signal used for detecting a load state at the power reception side.
FIG. 10 is a block diagram showing the configuration of a power transmission device of a power transmission system including the waveform monitor circuit disclosed in Publication No. 2009-33955. Referring to FIG. 10, the power transmission device includes a waveform monitor circuit 414 which generates an induced voltage signal used for detecting a load state based on a coil end signal which is a signal at one end of a primary coil L1 and is generated by load modulation at the power reception side and outputs the induced voltage signal, and a power-transmission control device 420 including a waveform detection circuit 430 for detecting a change in the waveform of the induced voltage signal input from the waveform monitor circuit 414. The power-transmission control device 420 detects the load state at the power reception side based on a result of detection performed by the waveform detection circuit 430.
The waveform monitor circuit 414 includes a current restricting (controlling) resistor RA1 provided between a coil end node NA2 at which a coil end signal CSG of the primary coil L1 is generated, and a monitor node NA11 at which an induced voltage signal PHIN1 used for detecting the load state at the power reception side is generated. The waveform monitor circuit 414 further includes a rectification circuit 417 with a limiter function which performs a limiter operation for clamping the induced voltage signal PHIN1 at a VDD electric potential and performs half-wave rectification of the induced voltage signal PHIN1.
The rectification circuit 417 includes a diode DA1 and a diode DA2. The diode DA1 is provided between the monitor node NA11 and a VDD node such that a direction from the monitor node NA11 toward the VDD node is a forward direction. The diode DA1 allows the induced voltage signal PHIN1 to be clamped at the VDD electric potential and a voltage which is equal to or higher than a maximum rated voltage to be prevented from being applied to an IC terminal of the power-transmission control device 420. By comparison, the diode DA2 is provided between the monitor node NA11 and a ground terminal such that a direction from the ground terminal toward the monitor node NA11 is a forward direction. The diode DA2 allows induced voltage signal PHIN1 to be half-wave rectified.
In the power transmission device of FIG. 10, a voltage reference of the primary coil L1 is not determined. Therefore, there are a case where the coil end signal CSG of the primary coil L1 becomes a positive voltage on the basis of a ground electric potential, and a case where the coil end signal CSG of the primary coil L1 becomes a negative voltage on the basis of the ground electric potential. In the case where the coil end signal CSG of the primary coil L1 changes from the positive voltage to the negative voltage on the basis of the ground electric potential, a forward current flows from the ground terminal toward the monitor node NA11 in the diode DA2. Thereby, an electric potential (electric potential of the induced voltage signal PHIN1) at the monitor node NA11 is lowered by an electric potential of a forward voltage (threshold voltage) VT2 of the diode DA2 with respect to the ground electric potential.
Every time the coil end signal CSG of the primary coil L1 becomes the negative voltage on the basis of the ground electric potential, the electric potential at the monitor node NA11 in the waveform detection circuit 430 is not clipped at a desired electric potential in half-wave rectification, is lowered by the electric potential of the forward voltage VT2 generated in the diode DA2 and becomes the negative voltage on the basis of the ground electric potential. Therefore, there exists a problem that the negative voltage is applied to the IC terminal of the power-transmission control device 420, thereby producing, for example, a parasitic effect (latch up, etc.) which causes breakdown of an electric circuit element included in the power-transmission control device 420.